Those skilled in the art are cognizant of a variety of methods for reclaiming gold from gold alloys with baser metals, gold-plated articles, and gold clad articles. The latter include articles referred to as "gold-filled" or identified as "rolled gold plate." in the United States, unless otherwise specified, both of those terms refer to articles made of base metals to which a sheet of gold not less than 10 kt in quality has been attached. The attachment may be to one or more surfaces by soldering, braising, welding, or other mechanical means. Articles designated as "gold-filled" must have a surface layer of 10 kt or purer gold wherein the karat gold is at least 120th of the total weight of the article. Articles having thinner coatings of gold may be marked "rolled gold plate." Watch cases are a specific exception in that the thickness of the gold cladding the article rather than its weight relative to the whole article is made the criterion for quality classification. In addition to gold recovery from scrap jewelry and other ornamental goldware, gold recovery from printed circuit boards and comparable electronic equipment is important. As used herein, a "base metal" is one more subject to oxidation and generally more reactive than gold. Gold, copper, or other metals in a zero oxidation state shall be referred to as being in a "metallic" or "elemental" form, in contrast to the reacted or oxidized form of a positive metal ion.
Gold-containing alloys, such as those used for jewelry, commonly contain significant amounts of copper and, in certain instances, nickel, silver, and other metals included to harden or modify the color of the resulting alloy. Watch cases and other decorative articles are commonly made of brass, bronze, or other base metal alloys that usually contain a large proportion of copper as well as zinc, tin, or other metals. More rarely, gold is applied as plating over or otherwise is used in conjunction with ferrous materials. In electronic circuit boards and the like, gold is commonly plated over copper and occasionally also extends over solders, which are predominately lead, tin, and zinc.
In many of the scrap articles of the sort just referred to, the base metals associated with the gold are partially or entirely deliberately covered by the gold, either for decorative purposes or because the gold is being applied for protective purposes, commonly to avoid corrosion or to enhance electrical conductivity. Similarly, though some of the base metal in gold alloys is at the surface, of course, the rest is distributed through the body of the article made of the alloy. As a consequence, before the base metals may be attacked chemically and dissolved, traditionally the gold must either also be dissolved or the base metal must be mechanically exposed in a manner comparable to pulverizing ores prior to hydrometallurgical treatment in order to allow access to the metals and minerals contained therein. Thus, in Ida, et al., U.S. Pat. No. 4,426,225, in which base metal is dissolved from beneath gold plate in printed circuit board scrap material, the nitric acid used as an etching medium for the base metal can act only upon such base metal as lies exposed at the margins of gold plated areas. It is stated in Ida, et al., that base metal located beneath gold plated areas becomes accessible to the nitric acid only after the gold of adjacent areas has flaked away as its base metal underpinnings are removed. Other past methods for recovery of gold from scrap generally involve an acid or cyanide reaction in which the gold to be recovered is dissolved, along with the other metals present. The gold is then isolated from the solution in a subsequent step.
A result comparable to that described in Ida, et al., is achieved by the method of Valentine, U.S. Pat. No. 4,261,738, in which base metal intimately bonded to gold is leached off with an ammoniacal solution of an ammonium salt using a leaching liquid containing reactive oxygen. Neither that method nor the Ida, et al., method is described in the references cited as effective to remove alloying metals from gold alloys, sometimes referred to herein as "karat" gold.
Various methods are known for removing metals from their ores, including bits of such metals in elemental form, by leaching with solutions containing oxidizing and complexing agents. Generally such methods require that the ore be pulverized at least before and sometimes during the leaching process. Such ores usually contain a large proportion of copper or other metals base with respect to gold. During such processes, both the gold and the base metals are dissolved before ultimately being recovered by chemical or electrochemical reduction. Examples include Chambers, et al., U.S. Pat. No. 3,692,647; Redondo-Abed, et al., U.S. Pat. No. 4,266,972; and Hougen, U.S. pat. No. 3,880,653. Reynolds, et al., U.S. Pat. No. 4,244,735, teaches a similar method applied to flue dust. With the exception of Chambers, et al., all of the methods referred to are carried out under an oxidizing atmosphere. Chambers, et al., teach the leaching of complex ores with a solution containing cupric chloride and sodium chloride. Chambers, et al., teach that the leaching should be carried out "under conditions which substantially prevent the copper from coming in contact with oxygen. The reason given for this is that, under the conditions therein described, the copper is "readily converted to insoluble oxychloride of copper." (Column 2 at line 20). Chambers, et al., teach as a method adequate to this purpose permitting the various solutions described in the Chambers, et al., process to contact the air so that a surface crust of copper and other metal oxides forms.
Those skilled in the art are not cognizant of a method for isolating gold in metallic form from gold-containing alloys, scrap metal partially or entirely covered by gold by electroplating or mechanical attachment and from scrap electronic components that include metal parts containing or plated with gold, the method being such that the reagents used in the method are not unduly hazardous to workers and the environment; leaching solutions are not consumed but instead may be regenerated in the process and recycled; gold is recovered in metallic form without the need for electrolytic or substantial chemical reduction; and gold-containing materials need not be pulverized or otherwise subdivided as a preliminary step to gold recovery.